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Fish Spawning

Female fish lay eggs, with the eggs being fertilized by the males in close attendance. Eggs develop more rapidly (in a few weeks) in warmer temperatures, and more slowly in colder water (up to months). Most freshwater fish spawn in the spring, although salmon, char, and some trout spawn in the fall. Water temperature is a key trigger to timing, although other factors, like day length, are likely involved.
Where fish lay eggs, however, depends on the species. Some deposit eggs on rocks, others on weeds or flooded brush. Nesting fish, like those of the sunfish family (e.g., bass and bluegill) sweep out pit-type nests in which to spawn, with males remaining on the nests for several weeks to protect eggs and newly hatched fry from predators. Walleyes expel their eggs into gravelly areas swept by current, while trout and salmon create crude nests in rocky current areas, although none of these fish guard the spawn.

Males and Females
While most male mammals are larger than their female counterparts, females of most freshwater fish species grow to larger average sizes than males. Differences between the sexes are most apparent near spawning time, when the females lay their eggs and males fertilize and often guard the nests, which is most often in the spring or early summer, depending on the species.